1. Field of the Invention
The present invention relates to an image reading apparatus and a control method thereof. In particular, the present invention relates to a sheet-through type image reading apparatus and a control method thereof.
2. Description of the Related Art
As an image reading apparatus that optically reads an image on a document, a platen-set type image reading apparatus that reads an image on a document placed on a platen glass and a sheet-through type image reading apparatus that reads an image on a document while documents are conveyed one by one have been conventionally used alone or in combination. The sheet-through type image reading apparatus has advantages including size reduction, low cost, low noise, speedup of image reading, and high productivity of printing, and thus, the sheet-through type image reading apparatus has been the mainstream in monochrome copiers and color copiers.
The sheet-through type image reading apparatus is configured such that an image reading position is fixed in place, that is, onto a transparent member (elongated reading glass) and a focal point of a reading optical system is obtained on an image surface of the conveyed document through the reading glass. Therefore, the apparatus is susceptible to a foreign substance such as dust adhered onto the reading glass and dust remaining on the reading glass, and a portion where the foreign substance block light leads to an image noise streak in the read image. In the case of a paper document, a trouble that a minute foreign substance such as a fiber and a filler such as calcium carbonate included in the paper adhere to the reading glass cannot be avoided.
As a configuration for reading both the front surface and the rear surface of a document by means of the image reading apparatus, a configuration having a mechanism for turning over a document in the apparatus, thereby turning over the document whose front surface has been read and conveying the document again to an image reading position to read the rear surface has been conventionally common. This manner of reading is referred to as single scan reading.
In contrast, demand for an image reading apparatus further including a reading unit and configured to read the front surface and the rear surface of a document at the same time according to the sheet-through scheme has been expanding rapidly in recent years. This manner of reading is referred to as dual scan reading. The dual scan reading allows double-sided reading with the shortest path. Therefore, as compared with the single scan reading, the dual scan reading has advantages including high reliability and reduction in failed conveyance such as a paper jam, in addition to high productivity and low noise.
In the dual scan reading, a lightweight and compact CIS (Contact Image Sensor) is commonly used. The CIS has a feature of shallower depth of focus than that of a CCD (Charge Coupled Device).
The conveyed document must be brought as close to the CIS as possible in order that the CIS having a shallow depth of focus can read the document. In a configuration in which the document comes into strong contact with the CIS, however, an adhesive foreign substance (an adhesive substance protruding from an end face of a portion bonded by tape or glue, correction fluid, an ink lump of a pen, an eraser and the like) adhered to the document are easily transferred to the CIS. The adhesive foreign substance transferred and adhered to the CIS is not easily separated.
In a reading mechanism such as the CIS, the amount of light emitted from a lamp changes due to repeated use over the years, and thus, the reading density changes. Therefore, shading correction for reading a white color reference sheet prepared in advance before document reading, and making a correction to achieve the intended reading density is commonly made.
When the CIS having the foreign substance transferred and adhered thereto reads the white color reference sheet during shading correction, a shading value at a position where the foreign substance adheres becomes abnormal. Therefore, the density of a pixel at this position is constantly different from the density at the other positions and image noise streaks occur. The noise streaks caused by the abnormal shading value occur in all jobs affected by the shading correction value, which leads to extreme deterioration of the image quality. For example, when shading correction is made for each job before reading, image noise streaks occur on all documents in the job.
As a technique of addressing this problem, Japanese Laid-Open Patent Publication No. 2004-048184, for example, discloses a technique in which a rotating body having a white reference sheet and a cleaning member is provided at a position where the rotating body faces a CIS, and the rotating body is rotated, thereby cleaning a surface of the CIS before shading.
According to the technique in this publication, however, there is a problem that image noise streaks still occur when a foreign substance is not completely removed from the surface of the CIS by the cleaning operation.
Furthermore, there is a problem that a flaw appears on the surface of the CIS or the durability of the cleaning member declines faster when the number of the cleaning operation increases. In particular, in many cases, a reading glass of the CIS is generally subjected to the antifouling coating treatment for coating the reading glass with a substance such as Teflon® having low surface energy in order to prevent an adhesive substance from adhering to the reading glass. Therefore, there is a problem that an increase in the number of the cleaning operation leads to peeling off of the coating.